Feeds: fix pull-to-refresh cancellation

Created by: jedmund

Summary

Pull-to-refresh on Tag feed, Bookmarks tab, and pushed Profile was surfacing URLError.cancelled ("Failed to Load / Network error: cancelled"). Home worked because it's the nav-stack root; the cancellation only reproduced on pushed/tab-root hosting controllers. SwiftUI was cancelling the .refreshable action Task before the network round-trip completed, and the cancellation propagated into URLSession.data(for:).

Changes

• IllustFeedViewModel.refresh runs the fetch inside Task.detached so caller-task cancellation can't reach URLSession.data(for:). Still awaits fetch.value to keep the spinner up through the round-trip. Also keeps existing illusts visible during the fetch and swaps atomically on success — matches Mail / Twitter UX and avoids a mid-gesture view-tree collapse.
• ProfileView.refresh no longer re-fetches userDetail. Profile metadata is stable; blanking it and re-awaiting loadUserDetail reproduced the same cancellation against a different endpoint and flashed "Failed to Load" over a perfectly valid header.
• TagFeedView / ProfileView now keep their scroll-reactive scrollOffset / showsCenterTitle state in a nested body subview, so scroll ticks no longer re-render the view that owns .refreshable. (Precautionary refactor; reduces body re-evaluation on scroll regardless.)

Test plan

• Home tab: pull-to-refresh still works (regression check).
• Bookmarks tab (iPhone + iPad): pull-to-refresh; grid reloads, no error banner.
• Bookmarks: toggle Public ↔ Private, then pull-to-refresh on the new feed.
• Search → tap a tag → pull-to-refresh TagFeedView; grid reloads.
• Detail → tap user row → pushed ProfileView → pull-to-refresh; feed reloads, header stays intact.
• Scroll-reactive chrome still animates: TagFeedView title pill on scroll; pushed ProfileView center pill reveals past ~185pt.
• Pagination still works on all feeds (scroll to tail loads more pages).

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